Existing castable, insensitive, energetic compositions generally have two distinct physical phases--a continuous phase consisting of a soft, rubbery binder and a discontinuous phase consisting of a hard crystalline explosive solid dispersed throughout the binder. Upon shock or mechanical loading, separation or dewetting of the binder and solid can easily occur causing a significant increase in sensitivity and a resulting increase in undesirable hazard properties.
Amorphous-type energetic compositions and emulsion-type energetic compositions have been developed to avoid the phase separation problem. Known amorphous energetic compositions are characterized by a single phase in which a polynitroaliphatic energetic solid is completely dissolved in a nitropolymer fuel to form a soft, jelly-like material. Although amorphous-type compositions exhibit little or no crystal character under X-ray diffraction, they are impact sensitive and have undesirable mechanical properties for most energetic material applications. Emulsion type energetic compositions are characterized as a solid solution in which a solid crystalline explosive phase is dispersed in a continuous solid binder phase. Ammonium Nitrate (AN) emulsion-type compositions are prepared by mixing an immiscible molten AN and molten binder with an emulsifier to form a stable emulsion which becomes solid upon cooling. Only limited numbers of AN eutectic mixtures melt at temperatures low enough to be useful, thus limiting the energy level of the resulting composition. Additionally, vigorous mechanical agitation is necessary to form the emulsion. The combination of high temperatures and vigorous mechanical agitation of a molten explosive always creates some concern for safety during processing.
Thermoplastic elastomers (TPE) are desirable as binders for composite propellants due to their ability to form composite propellants without chemical cross linking. Crosslinked propellants cannot be redissolved. Furthermore, they tend to become brittle with age. The TpE binders are soluble, permitting lowering viscosity of the polymer in solution. Oxidizer salts can be dispersed in the binder at lower energy and lower temperature. Furthermore, waste and obsolete propellant can be safely returned to its components by remelting rather than requiring burning or explosion to dispose of the material.